1. Field of the Invention
The present invention relates to a method of controlling a common bus, and in particular, to a method of controlling a common bus in a computer system, the common bus, a plurality of units each having a distributed arbitration function in which each unit arbitrates a contention or conflict of bus usage requests from the plural units connected to the common bus based on a priority associated with a respective unit identifier assigned to each unit.
2. Description of the Related Art
In a common bus control, there have been problems that the utilization efficiency of the bus is lowered due to an occurrence of a wait time and that the bus usage frequency is concentrated on a particular unit in association with a priority control. As a means to solve such problems, there has been known a method, for example, as described in the JP-56-97129, in which there is uniquely and independently provided on the common bus a common bus control circuit for sequentially changing over the priority levels of the respective units receiving common bus usage requests so as to assign the priority level to the units in a dynamic fashion, such that the alteration of the priority levels and arbitration of the bus usage requests are controlled by the common bus control circuit in a concentrated fashion.
On the other hand, there has been recently employed a common bus control method, such as a small computer system interface (ScSi) as a typical example, in which the arbitration of the bus usage requests is distributed to the respective units connected to the common bus. In this method, a unit desiring to use the bus, upon issuance of a bus usage request, drives a bit on the common bus corresponding to the unit identifier, so that the respective units monitor the bit to compare the bit with the identifier thereof, thereby effecting the arbitration.
According to the above method, since the priority levels for the bus usage right are determined by weights of the identifiers associated with the respective units, although there does not take place an occurrence of a wait time, there arises a problem that the bus usage frequency is concentrated on a particular unit.
In the distributed arbitrating method, however, since each unit is required to recognize the priority levels of all the units, it is difficult to effect the control for alteration of the priority levels independently of the units, which has been describe din the example above.